The FiiO E09K is strong enough to power my AKG Q701. The headphones sound "free" or "unchained" compared to a direct connection to my Macbook Pro, which of course does not deliver enough power for those big and demanding studio headphones.
I think there are not many competitors compared to the FiiO E09K in terms of "bang for the buck". I'm completely satisfied with the connections, and the big volume knob, which feels sturdy and allows a very accurate volume control. Overall, there is no audible static noise coming from the amp which means the built in components are pretty good. Build quality is also very high.